High-grade metamorphic rocks in southern Altai Range, SW Central Asia: their origings, tectonothemal [i.e.tectonothermal] evolution and tectonic implications

Abstract

The Central Asian Orogenic Belt (CAOB), the largest accretionary collage on the Earth, has a complicated and prolonged accretionary history which remains being highly debated. High-grade terranes were previously interpreted as Precambrian micro-continents which played a very important role during the evolution of the CAOB. However, some of their presumed old ages are challenged by recent high-resolution dating results which raise questions on their Precambrian origins.

The Chinese Altai and Tseel Terrane in the SW CAOB, two typical high-grade terranes occupy vital structural positions, feature various lithological elements and exhibit complicated deformation-metamorphism patterns, making them key areas in the reconstructing of the evolution of central Asia. However, their origins are not firmly constrained. Paragneisses were considered as Precambrian basements, but yielded detrital zircon ages predominantly between 440 and 580 Ma. The associated granitic gneisses and amphibolite gave crystallization ages at 420-463 Ma. Geochemical and zircon Hf isotopic data of paragneisses support that their protoliths may represent significant erosion products of arc rocks that were developed in a subduction environment. This feature is similar with that of the associated low-grade volcanogenic schists which probably represent immature sediments in an active margin. Detrital zircons from the paragneisses and schists show similar age patterns, supporting derivation from similar provenance. Accordingly, our data reveal that these high-grade terranes do not represent Precambrian microcontinents.

Moreover, the U-Pb age pattern for the detrital zircons, and some xenocrystic zircons from the associated granitoids, is comparable with the age patterns of the micro-continents and arc terranes in western Mongolia. The predominant zircon population of 440-580 Ma matches the widely distributed granitoids within the Neoproterozoic-early Paleozoic terranes in western Mongolia, while the minor Precambrian ages (>540 Ma) resemble those old rocks preserved in the Tuva-Mongolian (TM) block and its adjacent Neoproterozoic arc terranes. These features suggest that detrital and xenocrystic zircons more likely represent the detritus recycled from western Mongolia. Accordingly, the crustal growth of the SW CAOB in the early Paleozoic could be outlined by secular amalgamation of magmatic arcs around a Precambrian micro-continent. In addition, the TM-derived Precambrian zircons are further used to trace the origin of the TM block, which favors that the TM block was possibly rifted from the Indian block in the Neoproterozoic.

Further efforts have been made to decipher the controversial tectono-metamorphic history. In the Chinese Altai, U-Pb dating on the metamorphic zircon portions yielded consistent ages of ~390 Ma. Temperature estimations using mineral-pair as well as Ti-in-zircon thermometers revealed high-temperature conditions up to ~720℃. Detailed investigations on the metamorphic rocks in the Tseel area revealed that middle-pressure metamorphic fabrics developed under progressive NNE-SSW convergent setting, possibly at 385-374 Ma. A later low-pressure/high-temperature metamorphic sequence developed during decompression, associated with high-level anatexis at 374-363 Ma. Collectively, our data support that the final amalgamation of North Mongolian Domain on its southern margin occurred at Middle-Late Devonian, and might be immediately followed by the subduction of an active oceanic ridge.